Is it right if two christians (a boy and a girl) have a close (friendly) friendship relationship?
I do not really remember to have found such a relationship in the Bible. The Bible talks about friendship a lot, but it talks about the friendship between two men or two women, like Naomi and Ruth. Only one case where a man and a woman had a beautiful, trusty relationship is Esther and Mordecai. Anyway, it’s more about a relationship between a mentor and a disciple, plus that they were close relatives, so it’s a friendly relationship between close relatives.
Friendship involves self-sacrifice and sincerity. You can not be honest at times with someone of the opposite sex, because it is not appropriate or innocent.
Sooner or later somebody will begin to make illusions and friendship can take another course. Ultimately, this friendship will hurt you or your friend.
If you want to know how to have real friends, you can study the course on Abraham, which was named by God His own friend and the Covenant course.